The use of polymers in dispersions of photographic couplers and other photographically useful compounds is known in the art. Generally, polymer-containing dispersions are prepared with use of auxiliary solvents, i.e., volatile organic solvents or organic solvents with substantial water solubility. The polymer, coupler (or other photographically useful compound), and optionally other non-volatile solvent or hydrophobic components are combined with a volatile or substantially water-soluble solvent to form an organic solution. The organic solution is then emulsified in an aqueous medium, often containing gelatin and a surfactant, and the auxiliary solvent removed by evaporation or by washing the gelled dispersion with water. For either of these processes, ethyl acetate is often a preferred auxiliary solvent.
Photographic elements containing these polymer-containing dispersions may exhibit many advantages, including improved image preservability, improved physical properties, improved incubation storage before processing, and improved yellow leuco dye conversion.
The use of auxiliary solvent is important to the process of preparing polymer-containing dispersions. The solvent allows the coupler, polymer, and any other hydrophobic dispersions components to be combined in a mixed solution, so that a dispersion with an oil phase of uniform composition is obtained. The solvent also lowers the viscosity of the oil solution, which allows the preparation of small-particle emulsified dispersions. However, the use of auxiliary solvent also presents severe difficulties in the preparation of photographic dispersions and elements. First, the auxiliary solvent does not allow for the introduction of many types of polymers. Polymers of high molecular weight cannot be easily introduced, because the high oil-phase viscosity does not allow for the formation of small-particle dispersions, as discussed in U.S. Pat. No. 5,055,386 and EP 586,974. Crosslinked polymers cannot be introduced in this manner. Large amounts of auxiliary solvent and high mixing energy are often necessary to prepare small-particle dispersions with polymers of even modest molecular weight. A second difficulty with auxiliary solvent is that it can cause severe coating defects if it is not removed before the coating operation. Third, the steps of evaporating volatile solvent from an evaporated dispersion and washing a chill-set, washed dispersion leads to final photographic dispersions with variable concentration, so that careful analysis is necessary to determine the actual concentration of the photographically useful compound in the dispersion. Fourth, the volatile or water-soluble auxiliary solvents present health, safety, and environmental hazards, with risks of exposure, fire, and contamination of air and water. Fifth, the cost can be significant for the solvent itself, as can be the costs of environmental and safety controls, solvent recovery, and solvent disposal.
Direct dispersion processes avoid the use of auxiliary solvents. In one such process, the hydrophobic components desired in the dispersion, typically coupler and coupler solvent, are simply melted at a temperature sufficient to obtain a homogeneous oil solution. This is then emulsified or dispersed in an aqueous phase, often containing gelatin and surfactant. With appropriate emulsification conditions, small-particle dispersions of much less than 1 micron diameter are obtained by this process. The direct process also yields a dispersion with a known concentration of the photographically useful compound, based on the components added, with no variability due to evaporation or washing steps. No volatile or water-soluble organic solvents are needed, eliminating the hazards and costs associated with their use. The direct dispersion process, however, cannot be generally applied to the preparation of polymer-containing dispersions. Homogeneous molten oil solutions of most couplers and coupler solvents dissolve only limited amounts or types of polymers, even with low molecular weight. And soluble polymers increase the viscosity of the oil phase dramatically, so that small-particle dispersions cannot usually be prepared.
The use of latex or dispersed polymers in the preparation of photographic dispersions has also been previously proposed in the art. Usually these latex polymers are prepared by emulsion polymerization, although emulsified dispersions of organic-soluble polymers are also described. Loaded latex dispersions, in which a hydrophobic photographically useful compound is "loaded" into the latex polymer particles, are described in, e.g., U.S. Pat. Nos. 4,203,716, 4,304,769 and 4,368,258. The usual procedure for preparing a loaded latex is to combine a solution of the hydrophobic photographically useful compound in a water-miscible organic solvent with the aqueous latex. The resulting mixture, which typically has about a 1:1 ratio of water to organic solvent, is diluted with water or the organic solvent is removed by evaporation, with the result that the hydrophobic compound becomes associated with or dissolved in the latex particles. Variations on this procedure vary the order of addition of the organic solution and aqueous latex, substitute water-immiscible volatile auxiliary solvents for the water-miscible auxiliary solvents, incorporate the water-miscible organic solvent in the emulsion polymerization step, or require the formation of intermediate water-in-oil emulsions of the latex in volatile organic solvent before the formation of the final oil-in-water loaded latex dispersion. In some cases, photographically useful compounds are dissolved in the organic monomers prior to emulsion polymerization. Procedures are also described in which base-ionizable couplers and/or base-ionizable latex polymers are combined at high pH, often with auxiliary solvent present, followed by neutralization and/or addition of magnesium salts or alkaline-earth metal salts, to form a dispersion of coupler and polymer.
All of these procedures for preparing loaded-latex or latex-containing dispersions present severe practical difficulties. Rigid requirements exist for both the hydrophobic compound and the latex, especially for the procedures which use water-miscible organic solvent. In the initial mixture of hydrophobic compound, water-miscible organic solvent, and latex, the hydrophobic compound must not be precipitated by the aqueous environment, and the latex must not be coagulated by the large amount of organic solvent present. Many patents in the prior art describe a test for latex loadability, in which a suitable latex must not coagulate when mixed with an equal volume of the water-miscible organic solvent used in the dispersion preparation. Most latex polymers do not meet this requirement. A second problem with evaporated and washed dispersions is the manufacturing, environmental and safety concerns detailed above that result from the use of auxiliary solvents. Polymerization of monomers with photographically useful compounds dissolved in the monomers can cause free-radical destruction of the compounds and can impair the polymerization process, leading to unwanted crosslinking, or lowered polymer molecular weight, and to higher levels of residual monomer. None of the prior art describes procedures for loading latex polymers without the use of water-miscible or volatile auxiliary solvent at some point in the procedure. Additionally, it is often difficult or impossible to achieve high loading levels, i.e., greater than about a 1:1 ratio, of the hydrophobic compound or compounds in the latex, using the known methods.